1. Field of the Invention
The present invention relates to a hydro resistance anemometer for measuring velocity in a flowing fluid.
2. Description of the Prior Art
A variety of different types of electrically operated anemometers have been employed to measure fluid flow velocity. One type of electrical anemometer is a "hot wire" or "hot film" anemometer. In such an instrument a sensor is immersed in a flowing fluid in thermal contact with the fluid. The sensor contains an electrical resistance or thermistor element. A current is passed through the sensor, thereby creating heat due to resistance through the electrical element. The heat generated is partially dissipated by thermal conductance to the fluid flowing past the sensor. The rate of thermal transfer is dependent both upon the thermal conductivity and velocity of fluid flowing past the sensor. Since thermal conductivity in many flowing fluids remains essentially constant, changes in the resistance of the electrical element in a "hot wire" or "hot film" anemometer are caused mainly be changes in the velocity of flow of the fluid past the sensor.
In a fluid which is cooler than the electrical resistance element in the sensor, a decrease in resistance in the electrical element is indicative of a greater rate of thermal transfer to the flowing fluid, which in turn is indicative of a greater fluid flow velocity. Conversely, an increase in electrical resistance is indicative of a reduced thermal transfer to the fluid caused by a reduced fluid flow velocity. In "hot wire" and "hot film" anemometers no electrical current is passed directly through the fluid. Rather, the current is passed through a resistance element in the sensor which in turn transfers heat to the fluid by thermal conduction and/or convection. The resistance element in the sensor is applied as one input to an instrument for indicating fluid flow velocity. For example, the electrical resistance element may be one leg of a Wheatstone bridge.
Because there is a relatively great current flow through the electrically conductive resistance element in a "hot wire" anemometer, such instruments exhibit a relatively large power consumption. "Hot wire" anemometers are therefore unsuitable for use in applications where the fluid flow location is remote or unattended. Under each conditions a self-contained battery power pack for the sensor is necessary. Anemometers for oceanographic studies and surveys must often have self contained power supplies, as must anemometers employed in other remote and unattended locations.
An alternative type of electrical anemometer does not heat an electrical resistance element, but rather employs electrodes which are exposed to a flowing fluid in spaced proximity from each other. One such fluid flow velocity sensor is described in U.S. Pat. No. 3,148,541. The flow measuring device described in this patent has a bridge circuit, one leg of which includes a resistance probe. The probe is shaped with a conical tip to provide minimum hydrodynamic disturbance and a relatively high voltage signal is required across the bridge. The power requirement is approximately 10 watts. The probe employs a pair of electrodes, spaced approximately one millimeter apart, across which an ac voltage is applied. An electrical current is conducted through the fluid. The magnitude of this current creates a localized heating effect in the fluid. To the extent that the heated fluid is swept away by fluid flow, the resistance to the current through the fluid is varied. Accordingly, a high flow rate will rapidly dissipate the heated fluid and thereby lower resistance in current flow through the fluid. Conversely, a low flow rate will not rapidly dissipate fluid in the vicinity of the probe electrodes, so that the increased localized temperature in the fluid decreases resistance, for example, in sea water, to current flow through the fluid.